I have a 07 997 NB that I run SCCA SoloII on max tire size. Can I fit a Rival S 335 x 30 x 18 in the back with a 11 or 11.5 wheel.
On the front can I fit a Rival S 275 x 35 18 on the front with a 8 or 8.5 wheel. This will be a drive to the track and race then back home tire.

 

What offset do you have to run to fit the 275 in the front?

Any rubbing at all?

 

Why the desire for such wide tires? Improved grip?

 

There are guides for the maximum tire width for a given rim size online. Beyond there you are asking for trouble due to the sidewalls being pinched in way too far. This is especially true with a 30 aspect tire since there isn't much sidewall to allow for this.
8" max is a 245
8.5" max is a 255
11" max is a 305
11.5" max is a 315